Anomalous activity can occur in various contexts. In the context of online advertising and a pay-per-click advertising model, opportunities may exist for people to commit fraud, which can be viewed as anomalous activity relative to valid selections of online advertisements. For example, two competitors (A and B) may both purchase advertisements (“ads”) that are displayed on a website whenever a user searches for relevant keywords. A may wish to sabotage B by writing a software program that clicks on B's ads until B's daily ad budget is exhausted and only A's ads are shown to subsequent users.